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United States Patent |
6,064,913
|
Irlicht
,   et al.
|
May 16, 2000
|
Multiple pulse stimulation
Abstract
A stimulation strategy for cochlear implants seeks to approximate the time
domain response of a patient's neural system to electrical stimuli, to the
time domain response of a normal hearing person to a corresponding
acoustic stimulus. The strategy is designed to induce in the neurons of a
patient a time domain response to an acoustic signal which is similar to,
or approximates the time domain response induced by the normal processes
in a healthy person. Various implementations are disclosed.
Inventors:
|
Irlicht; Laurence (Brighton, AU);
Clark; Graeme (Eltham, AU)
|
Assignee:
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The University of Melbourne (Parkville, AU)
|
Appl. No.:
|
334823 |
Filed:
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June 17, 1999 |
Current U.S. Class: |
607/57 |
Intern'l Class: |
A61N 001/36 |
Field of Search: |
600/554
607/55-57
|
References Cited
U.S. Patent Documents
4532930 | Aug., 1985 | Crosby et al. | 607/57.
|
4536844 | Aug., 1985 | Lyon | 607/56.
|
4905285 | Feb., 1990 | Allen et al. | 607/56.
|
4947844 | Aug., 1990 | McDermott | 607/57.
|
Primary Examiner: Kamm; William E.
Attorney, Agent or Firm: Gottlieb Rackman & Reisman PC
Parent Case Text
This application is a division of application Ser. No. 08/817,481 filed
Apr. 16, 1997 and now U.S. Pat. No. 5,991,663.
Claims
We claim:
1. A method for producing sets of stimuli to be applied by a stimulus
generator to selected electrodes in an auditory prosthesis in response to
an electrical signal corresponding to an acoustic signal, said method
comprising:
processing said electrical signals in accordance with a predetermined
instruction set, said instruction set performing the steps of:
analyzing the electrical signal to determine the electrodes to be
stimulated;
for each electrode to be stimulated, determining a set of stimuli so that
the neural structures of a patient responsive to each electrode in
response to said set of stimuli have a time domain response which is an
approximation to the time domain response of a normal hearing person to
said acoustic signal, said stimulus set including a plurality of stimuli,
and each stimulus including at least the amplitude of the stimulus, and
the timing of the stimulus; and
providing control signals to said stimulation generator to cause said
stimulation generator to produce said sets of stimuli.
2. A method according to claim 1, wherein said step of analyzing further
comprises determining which of a predefined group of acoustic features of
a portion of the electrical signal corresponds to features of sets of
stimuli, wherein the stimuli required to be presented for a given
electrical signal are determined by successively analyzing said electrical
signal, and obtaining the appropriate control signals corresponding to
sets of stimuli.
3. A method according to claim 1, further comprising analyzing said
electrical signal within a plurality of frequency channels, and for each
channel performing a separate analysis corresponding to approximating the
time domain response of part of the neural structures corresponding to
that population of the nerve fibers most responsive to the frequency
channel.
4. A method according to claim 3, wherein said determining step is
performed using a model of neural response to determine an approximation
of the time domain response of a normal hearing person to a sample of said
acoustic signal, said approximation being used to derive a desired
stimulus amplitude.
5. A method according to claim 4, further comprising determining said
amplitude by calculating a desired stimulus function for the patient
corresponding to the approximation of the time domain response of a normal
hearing person, and deriving the desired stimulus amplitude.
6. A method according to claim 4, further comprising relating said desired
stimulus function to said desired stimulus amplitude using a look-up
table.
7. A method according to claim 1, wherein each electrode is associated with
neurons having a relative refractory period further comprising generating
for each stimulated electrode, a set of stimuli including multiple pulses
presented at a rate much faster than said relative refractory period of
the associated neurons.
8. A method according to claim 7, further comprising generating the
stimulus set for each electrode such that the adjacent neurons exhibit a
population per-stimulus time histogram which is an approximation to the
population per-stimulus time histogram generated by a corresponding
acoustic stimulus in a normal hearing person.
9. A method according to claim 8, further comprising using said instruction
set to determine the sets of stimuli for each stimulating electrode in
accordance with a technique selected from the set comprising a model of
neural response, predetermined patient response data, and telemetry from
said stimulating means.
10. A method according to claim 7, further comprising providing at least
some of the stimulus sets selected to excite different population bands of
neurons.
11. A method according to claim 10, further comprising using a size of the
population bands selected in accordance with experimentally derived
patient data in order to maximize the perception of sound by patients.
12. A method according to claim 11, further comprising using electrode
bands customized for each patient.
Description
TECHNICAL FIELD
The present invention relates to methods and devices for providing
electrical stimuli, and a strategy for electrical stimulation, for
auditory prostheses such as cochlear implants.
BACKGROUND ART
Cochlear implants of various types have been proposed and constructed. For
the purposes of explanation of the present invention, reference will be
made to arrangements such as are commercially available from Cochlear
Limited. However, it will be appreciated that the present invention is
equally applicable to other types of auditory prostheses. An intracochlear
electrode array is surgically implanted in a patient, together with a
receiver stimulator unit for providing electrical stimuli to selected
electrode pairs within the array. The receiver stimulator unit is
connected, via an inductive transcutaneous link or a direct percutaneous
connection, to an external sound processing device and microphone.
The present invention is concerned principally with the process of
selecting appropriate stimuli, and with the actual stimulus pulses
delivered in response to the acoustic stimuli. According to known
arrangements, stimuli may be applied between different pairs of
electrodes, to provide different modes of stimulation. In general, the
electrode pair selected is related to the pitch of a detected tone. In
this case, stimuli have generally used a consistent pulse timing and
shape, and amplitude is determined by reference to the amplitude of the
detected sound signal. It is also known to stimulate at a rate related to
a detected tone, so as to induce a pitch percept.
However, it has been determined that the perceptions of patients in
response to these stimuli are different from the perceptions using the
normal hearing mechanisms. It has been determined that, in particular, the
response of the auditory nerve to such stimulation is quite dissimilar to
the neural response of a normally hearing person to the same sound.
In a paper by Parkins et al, entitled "A fibre sum modulation code for a
cochlear prosthesis", Annals of the New York Academy of Sciences, 1983 at
p 490, the authors discuss providing stimuli in such a way as to mimic the
neural response of a normal hearing person to acoustic stimuli. The
stimulus waveform is modified, using a complex mathematical model, so that
the post stimulus time histogram approximates that of the normal hearing
case. However, the arrangement described is not suited for real time
processing so as to facilitate implementation in an implantable or
portable device.
U.S. Pat. No. 4,495,384 to Scott et al discloses a real time processing
arrangement for a cochlear implant. This disclosure does not describe a
system which accounts for the refractory period of nerve fibres, and as a
result the stimuli produced do not provide a neural response having a time
domain waveform similar to the normal hearing case.
In a paper by Motz and Rattay, (1988), "Signal processing strategies for
electrostimulated ear prostheses based in simulated nerve response", the
authors discuss the problems associated with hyperpolarisation of the
auditory nerve fibres, and consequent loss of perception of higher
formants by patients. The stimuli were simulated as if presented from a
single electrode. The authors propose the use of further pulses after the
initial stimulus pulse, the later pulses having considerable linear
increases in amplitude, to improve the perception of higher formants.
There is no disclosure of selecting pulses so as to produce a desired
post-stimulus time histogram in the auditory nerve structures.
It is an object of the present invention to provide a practical arrangement
for generating electrical stimuli so that an auditory nerve response is
produced Which better approximates the time domain response of the neural
structures of a normal hearing person to a given acoustic stimulus.
SUMMARY OF INVENTION
According to one aspect the present invention comprises a cochlear implant
device, comprising processing means for receiving an electrical signal
corresponding to an acoustic signal, and stimulation means adapted to
provide electrical stimuli to the cochlea of a human, said stimulation
means including an electrode array comprising a plurality of electrodes
operatively located within the cochlea, said device being arranged so as
to permit selected electrodes to be provided with stimuli, said
stimulation means being responsive to control signals received from said
processing means,
characterised in that said processing means processes said electrical
signals in accordance with a predetermined instruction set, said
instruction set determining the stimulation to be applied in response to
the acoustic signal including the electrodes to be stimulated, the
amplitude of the stimuli, and the timing of the stimuli, said processing
means providing control signals to said stimulation means to cause said
stimulation means to produce a set of stimuli, said set including for at
least one electrode a first stimulus pulse and at least one further pulse
within the refractory period of at least a substantial number of the nerve
fibres stimulated by said first pulse, the set being selected such that
the neural structures of a patient in response to said set have a time
domain response which is an approximation to the time domain response of a
normal hearing person to said acoustic signal.
The instruction set preferably analyses the electrical signal corresponding
to the acoustic signal, so as to identify portions of the signal as
corresponding to one of a group of predefined features. These may include,
for example, phonemes, tones or chords. A look up table may be provided
which provides stimulus sets which have been determined to operatively
induce an appropriate time domain neural response corresponding to the
acoustic feature. Alternatively, the neural response may be directly
calculated. This stimulation is then presented via the electrode array.
The sets of stimuli may be selected so as to stimulate selected populations
of nerve fibres at different times, so as to take account of the
refractory period of the nerve fibres. The sets of stimuli may also take
into account the responses of specific nerve fibres, and are preferably
tailored to the responses of a particular patient. For example, each
patient may have different degrees of array insertion, some electrodes may
not be active post-operatively, and different patients have different
degrees of nerve survival. The stimulus sets are chosen so as to allow the
nerve response to the stimuli to approximate the time domain response of
normal hearing patients to the respective acoustic stimulus. This response
may be determined with reference to the post stimulus histogram,
inter-spike intervals histogram, and/or the period histogram of individual
nerves, or of chosen bands or populations of nerve, or by other telemetry
from the stimuli.
The stimulus sets may be designed in advance, using preferably a model of
neural response and statistical and/or mathematical analysis.
Alternatively, the stimulus sets may be calculated in real time. The
stimulus sets may be calculated to take into account single unit effects,
or population effects, or preferably both. The present invention allows
tailoring of the stimuli, both for different acoustic inputs, and for the
peculiarities of individual patients. It is believed that the closer the
time domain waveform of the evoked auditory response via electrical
stimulation can be brought to that experienced by a normal hearing person,
the easier it will be for patients to interpret the precepts produced.
BRIEF DESCRIPTION OF DRAWINGS
The invention will be described with reference to the accompanying
drawings, in which:
FIG. 1 illustrates schematically an aspect of the theoretical basis fro the
present invention;
FIG. 2 illustrates in block form processing according to the present
invention;
FIG. 3 illustrates graphically theoretical and measured plots of spike rate
against stimulus function;
FIG. 4 illustrates a sample period histogram of a population of nerves;
FIG. 5 illustrates a multiple pulse histogram according to one embodiment
of the present invention;
FIG. 6 illustrates a histogram produced according to standard techniques;
FIG. 7 illustrates a histogram according to an embodiment of the present
invention;
FIG. 8 illustrates current levels required to produce the output of FIG. 5;
FIG. 9 illustrates the spread of action potentials excited by the
stimulation;
FIGS. 10 and 11 illustrate further implementation of the present invention
in block form; and
FIG. 12 illustrates schematically a cochlear implant system;
FIG. 13 illustrates the time and power signals associated with a phoneme;
FIG. 14 illustrates firing probabilities for various bands of neurons;
FIG. 15 illustrates the probability of spikes in various time periods for a
particular selected band; and
FIG. 16 illustrates a sample refractory function.
DESCRIPTION
The present invention relates to a broad principle for applying electrical
stimuli to patients with acoustic prostheses. It is emphasised that whilst
the present invention is described with reference to a specific
implementation, a wide variety of possible implementations exist. For
example, different models for neural response may be used to estimate the
required stimuli, and different stimulation arrangements, for example
percutaneous connection, may well be used.
A cochlear implant system of the type contemplated is illustrated in FIG.
12, and comprises in general terms a microphone 1 which receives sound
signals and passes a corresponding signal to the speech processor 2. The
speech processor processes the received electrical signal to produce a set
of stimulus data. This is transmitted, together with power, from the
external coil 3 to internal coil 4, and then to the receiver stimulator
unit (RSU) 5, which then provides stimulus pulses to the selected
electrode pairs of electrode array 6 so as to stimulate the nerve fibres
and provide a percept of sound to the user.
The neural response cannot be derived as a trivial function of, say, the
input sound signal. One aspect of the difficulty of accurately simulating
the response relates to the complexity of the system. The normal hearing
ear has approximately 30,000 nerve fibres, each of which can reach action
potential independently of the others at any time during the stimulus. It
is not possible to generate electrical pulses which cause this system to
behave identically to the normal hearing situation.
One aspect of the implementation of the present invention relates to
varying the numbers, amplitude, shape and rate of the pulses to evoke an
approximation of the NHNR. This is achieved in particular by evoking the
correct number of action potentials (counted across either one
representative fibre, or alternatively across the population of fibres)
within each "phase" of the waveform, or averaged over many appropriately
chosen phases of the waveform. This phase corresponds to the available
divisions within the stimulation period, which is limited by the
modulation rate--if the modulation rate was 4 times the frequency of a
tone, then each "phase" would be one quadrant of the waveform. This
results in a "staircase" approximation to the NHNR which is very similar
to that of the acoustical case. The specially designed stimuli are applied
across each period of the simulated tone, and are designed via computer
simulation, and mathematical analysis, of the auditory nerve's response to
both auditory and electrical stimulation.
Modifications to the waveform are used to improve the spatio-temporal
neural response. These modifications include (but are not limited to)
modifications to the number, amplitude, spacing, and width of the pulses
to better simulate the spatio-temporal pattern of the NHNR. These degrees
of freedom are provided in conventional systems, but are generally not
exploited.
The present invention allows for various types of temporal response to be
induced, as is required in various applications. One approach is to
utilise the present invention to evoke as near an approximation as
possible to the correct population per stimulus time histogram, by
applying several pulses per stimulus period. The pulse amplitudes may be
chosen so as to generate the correct number of action potentials in each
part of the waveform. The pulse sizes may be chosen using various means,
examples of which are discussed below.
A further aspect of the present invention is that by utilising the
refractive properties of neurons, it is possible according to the present
invention to provide stimuli such that different bands within the
stimulation range of an electrode (SRE) fire at different times. This
allows for the stimuli to evoke desired inter-pulse timings within each
band, so that the phase relationship between the bands in normal hearing
can be approximated. The size of the bands chosen may be selected, so that
the band size may be selected to be that size which provides the best
percepts for the patient. This may be customised for each patient.
The present invention is described largely in the context of available
implant systems, which utilise a standard biphasic pulse. Altering the
pulse shape will necessarily alter the time domain response of the
associated neural structures. The present invention is not limited in
scope to the use of existing or standard pulse shapes, although clearly
alternative pulse shapes will alter the details of the effects noted
above.
In order to more fully understand the present invention, we will initially
briefly consider its theoretical basis. In a conventional cochlear
implant, the neurones of the auditory nerve are stimulated by application
of a series of biphasic currents between electrodes of the electrode
array. Each biphasic stimulation causes a group of neurones to fire. The
number of neurones that fire due to a stimulation is determined by such
factors as relative location of the group of neurones to the stimulating
electrodes, and the history of stimulation of those neurones. If many of
the neurones are in a refractory period due to past stimulation then the
application of new stimulation will not cause as many of them to fire as
would be the case if they were being stimulated for the first time.
It is further desirable according to the present invention to provide an
estimate which is representative of the temporal response of the wider
population of nerves, not merely those close to a single electrode. In
order to overcome variations in neuron response due to location with
respect to the stimulating electrodes, the neurones can be considered as
divided into strips, each of which are assumed to contain neurones that
are equally stimulated by the application of a given stimulus pulse. This
is illustrated schematically in FIG. 1. Neurones 12 in the region of
electrodes 10, 11 are notionally divided into strips, labelled i, i+1, and
so forth.
Suppose that a single biphasic stimulation is applied between electrodes
10,11, at various amplitudes, and the ith strip of neurones monitored.
Whilst of course in practise any given pulse will stimulate multiple
strips, it is assumed that this strip contains the neurones most
responsive to the stimulating electrode. The stimulus function S.sub.k
describes the neural response from the kth pulse. A.sub.k is the amplitude
of the kth pulse. It is possible to make up a table for each strip
relating A.sub.k to S.sub.k.
In practice the neurones are not stimulated by isolated biphasic pulses but
by a series of stimulations. Each electrical stimulation will elicit a
neuronal response from a single strip of neurones of NI.sub.k action
potentials, where N is the number of neurons in that strip, and I.sub.k is
the averaged probability of any neuron from that strip acheiving action
potential during pulse k. It is known that the pulse in a series of
stimulation pulses that elicits I.sub.k has the same amplitude as the
isolated pulse that elicits S.sub.k where S.sub.k and I.sub.k are related
by:
##EQU1##
Let the pulse period equal T. (n+c) is the length of the relative
refractory period divided by T, and .gamma.(k-i) is one minus the
refractory function measured for time kT since the last action potential.
Therefore it is possible to generate a particular I.sub.k by determining Sk
and then looking up the appropriate amplitude of the biphasic pulse
A.sub.k to be applied.
So far it has been shown how to determine the amplitude of the biphasic
stimulation to be applied in order to elicit a desired neural response
I.sub.k in the ith stimulated strip of nervous tissue.
Linking of Acoustic Signals to Neural Response
Current speech processors used in cochlear implant technology rely on
extracting significant features of speech. For example, using the SMSP
process, electrical signals corresponding to received sound signals are
processed by means of band pass filters, eg. 16, to provide a signal
corresponding to amplitude in each channels. A selected number of said
amplitude signals having the greatest amplitude, e.g.6, are used to
modulate the amplitude of the stimulation pulse.
In order to incorporate the present invention into such a system it is
necessary to calculate the I.sub.k which would arise in the normal hearing
situation in each band wherestimulation is to occur. This I.sub.k may be
calculated by use of an approximate model of the cochlea and normal
neuronal response. See for example Parkins et al "A Fibre Sum Modulation
Code for a Cochlear Prosthesis", Annals of the New York Academy of
Sciences, 1983 p490, or one of the many other published models.
The I.sub.k is then mapped to the appropriate S.sub.k by means of the
equation above, and the map of A.sub.k to S.sub.k is used in order to
determine the amplitude of the biphasic pulse to be applied. This process
is described in FIG. 2. An input signal 20 is processed by software 21 in
order to extract a particular feature or set of features. This process may
be a conventional cochlear implant type, for example SMSP or
identification of formants. Alternatively, it may be a software process to
recognise phonemes or similar features, such as discrete musical tones.
The recognised feature is referenced via look-up table 22 to provide a
desired normal hearing neural response, I.sub.k, which corresponds to a
percept of the feature extracted. S.sub.k can then be determined with
reference to the equation above. An amplitude A.sub.k for each pulse can
then be derived from look-up table 24. Simultaneously, according to this
implementation, the input signal 20 is processed 25 so as to select an
electrode pair for tonotopic stimulation. The stimulus is then determined
26 by combining the derived A.sub.k with the electrode site selected at
25, to provide a set of stimuli to electrode array 27.
The look up table may be provided using any conventional memory device. The
first table stores the required type of patient percept, that is, the
feature extracted (e.g. a phoneme, or a tone), with corresponding normal
hearing neural response patterns. The other input to the table is the
required volume level of the perception. The output of the lookup table is
a set of electrical stimuli which evoke the desired neural response. These
are preferably calculated off-line via methods similar to those described
above, and stored. This arrangement allows for a reduction in processor
capacity, as it is not necessary for whole waveforms to be fully
calculated.
The second lookup table 24 requires as input the width of the stimulation
pulses, the rate of stimulation, and the desired stimulus function
(S.sub.k), and returns the amplitude A.sub.k of the stimulus required for
this. The values for the lookup table may be obtained in a variety of
ways. One approach is to use animal studies with a variety of pulse rates
at a variety of amplitudes and rates (for each pulse width). From the
responses measured, the `s` function can be calculated.
FIG. 3 lists the expected spike rates for biphasic pulses as a function of
the S function (for given pulse rates and pulse width) versus the actual
experimental results. The experimental results were obtained by putting in
fixed width biphasic pulses at various pulse rates and intensities, and
graphing the neural response rates. The theoretical values may be
calculated as follows.
Assume the stochastic process describing the timing of the action
potentials is a self exciting point process (Snyder and Miller, 1991).
Define the number of spikes (events) to time t as Nt ,then at any time t,
the time since the last spike equals t-t.sub.Nt. The intensity of the
point process (Snyder and Miller, 1991) is equal to s(t)r(t-t.sub.Nt),
where s(t).gtoreq.0 is a stimulus related function, depending on time
(determined by the properties of the neuron and also the signal presented
to the neuron) and r(.).gtoreq.0 is a refractory function, which lowers
the rate of action potential generation as a function of the time since
last action potential. r(.) is determined solely by the properties of the
neuron, and possibly also by the type of stimulus (electric or acoustic),
and is independent of the size of the stimulus.
Consider a system where the s function is a set of identical pulses spaced
at a period of T with the width of each pulse W being less than the dead
time of the neuron. Let the refractory function r(t-t.sub.Nt) be constant
over the following regions:
##EQU2##
Define
##EQU3##
A is the probability of there being no points assuming a Poisson rate of
s(t) during the pulse, and no refractory effects. .alpha..sub.n is the
size of the refractive function, where the last action potential occurred
n pulses ago. Define N as the smallest integer such that
(N+1)T-W.gtoreq.b. Then, the steady state average rate of neural firing
equals:
##EQU4##
The values for lookup tables which relate the S function to the electrical
intensity, for given conditions, may then be derived. This may be done in
a number of ways.
A relatively simple method involves simply measuring the `S` function
directly for a given pulse set up by measuring the neural response under a
number of conditions of pulse rate and intensity.
For instance, from FIG. 3, a pulse at an intensity of about 35 when
presented at 200 pps would equate to an S function of about 10, and
increases in intensity will approximately relate to increases in S
function in a linear plus offset relationship.
Alternatively, at 200 pps, to produce an S function of, say 20, a stimulus
intensity of about 40 is required.
Of course, further research may result in a more detailed representation of
the relationship, but this simple initial approach provides a reasonable
representation.
An alternate method of determination of the necessary current values for a
given pulse rate and width would be to (with each patient) apply a series
of pulses at fixed rate and width, and determine the threshold and
comfortable levels of current. Then, a second parameter which indicates
the size of the effective `S` function at each level could be determined
either by masking studies, or alternately by experiments where such a
parameter is changed, and the perceptual response noted, ie. a particular
sound could be coded, and then repeatedly played to the patient, under the
assumption of a given proportionality between the current intensity, and
the S function. The proportionality which returns the `best`
response--either in terms of naturalness or in terms of signal
discriminability, could be stored in the look up table.
Thus, in this particular implementation, there would be three parameters: a
threshold and comfortable level current, and a scalar parameter relating
the current intensity (at a given pulse rate) to size of the `S` function.
Note that this would need to be done for each electrode stimulation
combination. (ie. monopolar on each, bipolar on each pair, etc).
It will be appreciated that the pulse timing may be determined in various
ways, within the scope of the present invention. In a simple
implementation, a constant pulse rate may be used for all electrodes. This
rate must of course be much faster than the relative refractive period,
typically 20 ms, and is preferably less than 1 ms. A preferred
implementation uses a pulse rate for each electrode such that the rate is
an integral multiple of the characteristic frequency of the adjacent
neural population.
FIG. 10 illustrates in block form an alternative implementation of the
present invention. In this case, the received acoustic signal is processed
by a transducer, and then enters a filter bank with n outputs.
Illustratively, this may be 6. For each channel, a model of neural
response for that part of the neural structure is used to produce a normal
hearing neural response (NHNR) for that part of the acoustic signal
falling within the channel. The S.sub.k can then be calculated using the
equation shown above. The S.sub.k can be related to A.sub.k in a look-up
table, as previously discussed. This A.sub.k can then be used as the basis
for an instruction to the RSU to stimulate the appropriate electrode pair
at amplitude A.sub.k.
FIG. 11 illustrates a related implementation to FIG. 10. The distinction is
that for each channel output from the filter bank, FFT techniques are used
to derive a fundamental tone. Using a similar process to that described in
relation to FIG. 1, this tone is related to a NHNR via a look up table,
the S.sub.k calculated, and the corresponding A.sub.k determined from a
further look-up table. A stimulus instruction is then sent to the RSU
based on the determined A.sub.k, and the electrode site corresponding to
the tone. This process may be performed for each channel, or for a
selected set of channels, determined via the SMSP technique, which have
the greatest amplitude.
FIGS. 8 and 9 illustrate the principle of the present invention. To cause a
population response similar to that from a NHNR for a 1 kHz tone, we could
apply a continuously repeating set of four .mu.s biphasic pulses, where
the amplitudes were in the ratio of 4,6.5,7,0, resulting in a neural
response in the ratio of 10,24,10,0. This is illustrated in FIGS. 8 and 9.
The stimulation strategy proposed according to the illustrative example
below is designed to be capable of implementation on a speech processor
for cochlear implants which codes signals in terms of biphasic pulses. The
examples illustrated utilise a fixed-width biphasic--bi-polar pulse, with
an overall pulse width of 250 us.
FIG. 4 illustrates a population histogram for a population of 64 nerves
around the 1 KHz place in a cat cochlea of total length 2.5 cm. The
acoustic input is a 1 KHz tone. FIG. 5 illustrates an approximation using
multiple pulses according to the present invention.
FIG. 6 illustrates output pulses using one pulse per period, in other
words, using standard stimulation techniques. It is clear that such
fixed-rate stimulation techniques can not form a close approximation to
the desired output histogram at any frequency other than that of
stimulation.
A multiple-pulse electrical stimulation model was iterated in a trial and
error fashion until a set of current levels was found which provided the
required histogram according to the present invention. The results can be
seen in FIG. 7. It is clear that for each period, the actual histogram
closely resembles the desired approximation of FIG. 5. Clearly, when
compared to FIG. 6, the present invention provides a much closer
approximation.
EXAMPLE
The following describes the implementation of the inventive techniques in
relation to a specific sound input.
The phoneme /e (sounds like a short `eh`) is shown in FIG. 13,together with
its power spectral density. Note that the spectrum has a number of peaks
in the frequency spectrum (at about 800, 500, and 200 Hz). These may be
used as the main frequencies targeted for stimulation. The signal is from
an isolated sound.
It was applied to the model from Benjamin D Brayant and John D Gowdy,
"Stimulation of Stages I and 11 of Seneff's Auditory Model (SAM) Using
Matlab", published in the proceedings of the 1993 Matlab User's Group
Conference.
The model provides the averaged neural response for neurons from forty
regions of the basilar membrane (ranging in characteristic frequency from
high frequency to low frequency). Of course, the model could be set for
any number of bands required, for example, the response of bands
corresponding to each stimulating electrode. The response of some of the
bands are shown as FIG. 14. It will be appreciated that other models and
software could be used to produce this result.
The inventive technique may be used to code each band which corresponds to
the characteristic frequency of neurons close to an electrode. For now,
let us imagine that band 40 (for instance) corresponded to an electrode,
and examine how the inventive technique could be used to generate pulses
for that electrode. It will be appreciated that other electrodes would
also be coded at the same time.
In the neural response here, there are two aspects. A broad lowering of
probability over time, probably due to onset effects, as well as a fine
structure. The present invention provides information to the user about
both.
The fine structure of response here has approximately 20 periods in the 80
ms, corresponding to a period of 4 ms, or a frequency of 250 hz. As an
example, we will code with 8 pulses per period, requiring a coding
frequency of 2000 Hz, or in other words a bin size of 0.5 ms.
The probabilities shown in the graph will be the I.sub.k, or NHNR, of the
theory above. So using the formulas given, it is possible to work out the
s.sub.k which will give the required responses.
Let us assume that the population of neurons we wish to control can be
approximated by 3 approximately equally stimulateable regions (the centre
one being the most stimulateable), and we want the total number of action
potentials from this summed population (divided by the total number of
neurons in the summed population) to follow the curve of the Figure.
Use the following equation,
##EQU5##
where S.sub.k,i is the stimulation function for the ith regionduring pulse
k, I.sub.k,i is the averaging probability of neural response for the ith
region during pulse k, and .gamma.(k) equals one minus the refractory
function evaluated for the case where the last action potential occurred
k.T ago
The probability of firing in each 0.5 ms bin, can then be calculated, as
shown in FIG. 15.
From this calculation, the probabilities required for the first 10 bins
are:
______________________________________
k l.sub.k
______________________________________
1 .014
2 .000
3 .000
4 .022
5 .371
6 .382
7 .231
8 .001
9 .000
10 .000
______________________________________
A given electrical pulse will elicit different responses at different
distances from the site of stimulation. .alpha..sub.i is defined as the
ration between a nominal S for some pulse, and the actual S generated for
the ith region. Let us assume that the .alpha..sub.i for the three
sub-populations are 0.7, 1 and 0.7 for each population (1, 2, 3).
FIG. 16 gives approximate values of the refractory function(1-.gamma.), and
the gammas would be (very approximately), about 1 for the first three bins
(1 ms), and decreasing from there to about zero after about 25 ms (the
50th) bin. Therefore, responses from the last 50 bins are relevant when
calculating the response in any bin.
So gamma will be approximately 1 for the first two bins, and 0.5 after bin
10 or so, and then 0.97 after bin 45, etc.
To obtain the required population per-stimulus time histogram from the
total of the 3 sub-regions, we apply the formula:
##EQU6##
So, to do the calculation:
1. Assume that before time 0 there has been no significant amount of
firing. (If a previous token was coded earlier, then the processor will
remember the I.sub.k 's from that token, and how long ago).
2. Loop over each pulse, or `k`.
3. Calculate G.sub.k,i. as described above.
4. Using the equation above, calculate the required S.sub.k. Also, store
the three I.sub.k,i values this will evoke for use in calculations of
future G.sub.k,i values.
5. Using the lookup table (as previously discussed) determine the required
intensity of the pulse.
6. Administer the pulse for the correct amount of time, and go to step 2.
This procedure is readily implementable using conventional software
techniques.
Variations and alternatives are possible within the general scope of this
invention, as will be apparent to the reader. In particular, it is noted
that the various processing components may be differently arranged, so
that for example some or all the look up tables are located within the
implanted portion of the device.
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